The invention concerns a multiple-axle steering system preferably for agricultural machines, such as harvesters or tractors, having at least one primary-controlled axle and at least one other axle.
Multiple-axle steering systems of the generic kind are used where different applications of agricultural machines make it necessary to adapt the properties of the machine's maneuverability and tracking capability to the environment within which the machine is operating. These increased demands can be met by various types of steering systems described briefly below. The primary-controlled axis of a generic multiple-axle steering system is usually controlled hydrostatically or electrohydraulically.
Many agricultural machines have a front-axle steering system in which only the front wheels are turned, while the rear wheels remain in the straight-ahead position. This type of steering is needed particularly for road driving.
In addition some agricultural machines have a type of steering referred to as all-wheel steering in which the front and rear wheels are turned in opposite directions. All-wheel steering is preferable where movements of the machine are to be performed in a small space. The turning of the front and rear wheels in opposite directions improves maneuverability of the machine and is particularly effective for making small turning circles of the machine.
Another type of steering is synchronous steering in which the front and rear wheels are turned in the same direction. This type of steering is particularly useful on steeply sloping terrain, where a track parallel to the slope must be maintained. The front and rear wheels turning in the same direction preclude slipping of the machine on the slope or an undesired change in direction of the machine. Synchronous steering is also used on terrain where the lowest possible soil loading or low soil compaction is desired. In this case the front and rear wheels of the vehicle are deflected in the same direction through a relatively large angular range--an angular range of 15.degree. is quite normal here. The four or more wheels of the machine produce, due to the large deflection in the same direction, a number of closely adjacent tracks corresponding to the number of wheels, thereby avoiding the problem of increased soil compaction generally caused by the load of the several wheels in one track. Synchronous steering is sometimes called crab steering.
It is easy to imagine that the different types of steering described above impose high demands on the operator of the machine. It is also necessary to consider particular operational aspects which arise when changing between the individual steering modes, and also when using multiple-axle steering at different driving speeds of the machine. Typical all-wheel steering systems are designed so that the steering deflection of the rear wheels corresponds to that of the front wheels in the opposite direction. Depending on the speed of the machine and the steering movement, this can lead to rapid unexpected changes in magnitude of steer and of direction of the machine. This special problem increases the risk of erratic steering, particularly in the case of unskilled machine operators.
It is an object of the present invention to overcome one or more of the problems described above. It is another object to eliminate the disadvantages of generic multiple-axle steering systems described above, particularly with the all-wheel steering mode. Still another object is to provide a multiple-axle steering system in which different types of steering are combined into a common steering system or strategy. It is desirable that the steering strategy minimize the need for special training of the machine operator and the need for special experience in operating machines which utilize multiple axle steering systems. It is also desirable that the steering strategy minimize uncontrolled and unexpected steering deflections.